Process and system for control of fluids in water disposal surge tanks

ABSTRACT

This specification discloses a water-disposal process and system which includes a surge tank in which oil is automatically withdrawn from an oil phase on top of the water at a rate to maintain a constant oil blanket on the water. The oil is withdrawn through a conduit which is secured to a buoy which floats at the oil-water interface. The buoy thus supports the oil inlet of the conduit a fixed distance above the oil-water interface to maintain the thickness of the oil phase constant as the water level varies within the tank. The tank further comprises means for alternately filling and emptying the tank of water between predetermined upper and lower levels. The sides of the tank between these levels are thus contacted with the oil phase to provide corrosion protection.

United States McKee atent [451 July 11, 1972 [72] Inventor: Horace L.McKee, Corpus Christi, Tex.

[73] Assignee: Mobil Oil Corporation [22] Filed: May 23, 1969 21 Appl.No.: 827,224

Primary Examiner-Reuben Friedman Assistant Examiner-T. A. GrangerAttorney-William J. Scherback, Frederick E. Dumoulin, William D.Jackson, Andrew L. Gaboriault and Sidney A. Johnson [ ABSTRACT Thisspecification discloses a water-disposal process and system whichincludes a surge tank in which oil is automatically withdrawn from anoil phase on top of the water at a rate to maintain a constant oilblanket on the water. The oil is withdrawn through a conduit which issecured to a buoy which floats at the oil-water interface. The buoy thussupports the oil inlet of the conduit a fixed distance above theoil-water interface to maintain the thickness of the oil phase constantas the water level varies within the tank. The tank further comprisesmeans for alternately filling and emptying the tank of water betweenpredetermined upper and lower levels. The sides of the tank betweenthese levels are thus contacted with the oil phase to provide corrosionprotection.

3Claims,2Drawingfigures 882,195 3/1908 Gran ...2l0/242 2,661,094 12/1953Stewart ..2l0/242 200% l l l l l 22a 1 I- E 24 1 its PATENTEDJUL 1 1 m2HORACE L. MCKEE INVENTOR *Wmz/m ATTORNEY PROCESS AND SYSTEM FOR CONTROLOF FLUIDS IN WATER DISPOSAL SURGE TANKS BACKGROUND OF THE INVENTION Thisinvention relates to the control of fluids in the surge tank of a waterdisposal system and more particularly to the regulation of the oil phasewithin the tank to maintain a constant blanket of oil on top of thewater and yet prevent withdrawal of such oil from the tank with thewater as it is directed to a water disposal zone.

It is oftentimes necessary to dispose of large quantities of water. Forexample, in many oil fields large volumes of water are produced with theoil from the wells. This water often has a high salt content andnormally cannot be allowed to drain into fresh water surface streamsbecause of the problems presented by contamination. Thus, oil producersare compelled to dispose of such water by techniques which will notdamage the surrounding area. The water may be drained into storage sumpsor ponds where it evaporates or seeps into the subsoil. Alternatively,the water may be injected into wells leading to subterranean formationsbelow the fresh water level.

In distributing the water to the appropriate disposal zone, the waternormally is first accumulated in a surge tank. The water then iswithdrawn from the surge tank, either continuously or intermittently,and passed to the disposal zone. Several problems are attendant to theuse of such surge tanks. Corrosion of the tank is severe, particularlywhere the water contains dissolved salts. In order to protect againstsuch corrosion, it is a common practice to maintain a gas blanket of anonoxygen containing gas, e.g., natural gas, over the water. While thistechnique has met with some success in alleviating corrosion, certainsafety hazards are present since the tank must be a closed systemwithout vents to the atmosphere. In addition, the water to be disposedof often contains small amounts of oil. This oil because of itsrelatively low density rises to the top of the water where it forms aseparate oil phase. Over a period of time, particularly if the treatingsystem upstream of the surge tank fails to operate properly such thatthe water has an unusually large oil content, the oil phase may increasefrom a desired thickness of several inches to several feet or more. Thisexcess oil should be bled off in order to prevent withdrawal of the oilwith the water since itusually will be desirable to avoid introducingthis oil into the water disposal zone. For example, where the disposalzone is a subterranean formation, the introduction of oil into theformation with the water may result in a decrease in the relativepermeability of the formation to water. In some cases, this permeabilityreduction may be so severe as to plug the formation, thus making thewell unsatisfactory for water disposal purposes.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a new and improved process and system for the disposal ofwater containing small amounts of oil. The water is introduced into asurge tank where it separates into a lower water phase and an upper oilphase in accordance with conventional practice. As water accumulateswithin the tank, it is withdrawn from the tank under conditions suchthat the height of the interface between the oil and water phases varieswithin the tank. This is accomplished by intermittently allowing thetank to fill with water to an upper level therein and then withdrawingwater from the tank at a rate sufficient to lower the interface to alower level within the tank. In addition, oil is separately withdrawnfrom the oil phase within a tank at a location above the oil-waterinterface and at a rate such that the thickness of the oil phase remainsconstant as the height of the oil-water interface varies within thetank. By this technique, an oil blanket is constantly maintained overthe water in order to alleviate corrosion of the tank and, in addition,oil is automatically drained from the tank in order to avoid a large oilaccumulation therein which is subject to withdrawal with the water tothe water disposal zone.

In a further aspect of the invention, there is provided a preferredsystem which may be used to carry out the abovedescribed method. Inaddition to the means by which the water is withdrawn from the surgetank, the tank is provided with a conduit which extends from theinterior to the exterior of the tank. This conduit has an inlet in fluidcommunication with the interior of the tank such that oil may beseparately withdrawn through the conduit. The system further comprisesmeans for maintaining the conduit inlet at a fixed distance above theinterface between the oil and water phases. Such means comprises a buoyto which the conduit is secured. The buoy has a density between thedensities of oil and water and is adapted to float at the interfacebetween the oil and water phases to support the conduit inlet above theinterface. Thus, the thickness of the oil phase remains constant eventhough the elevation of the water column within the tank varies.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration partly insection showing a water dis- DESCRIPTION OF SPECIFIC EMBODIMENTS Withreference to FIG. 1, there is illustrated a water distribution systemembodying the present invention. The system comprises a surge tank 2equipped with a water inlet line 4, a vent 5, a water outlet line 6, andan oil outlet line 8. The tank 2 is shown as partially filled with acolumn of water 9 and a layer of oil 10 which floats on top of thewater. In accordance with the present invention, the tank also isequipped with a buoy 12 of a density such that it floats at theinterface 14 between the oil and water phases. A conduit 16 is connectedto the buoy and extends to the oil outlet line leading from the tank.The conduit 16 has an oil inlet 18 adjacent the upper end thereof. Asoil accumulates in the oil layer it flows into this inlet and drainsthrough conduit 16 into the oil outlet line 8 which forms a continuationof this conduit.

The oil thus withdrawn from the surge tank is passed through line 8 to asuitable oil collection zone such as a storage tank (not shown). As theoil is accumulated in this tank it may be withdrawn therefrom and passedto a treating facility. In order to the passage of gas between the surgetank 2 and the oil storage tank, it is preferred to provide line 8 witha gooseneck as indicated in FIG. 1 by reference character 8a. The liquidseal within gooseneck will prevent the passage of air through conduit 16to the storage tank or, if the surge tank is a closed system employing anatural gas blanket, the loss of the gas blanket to the storage tank.

In operation of the system shown in FIG. 1, water is introduced into thetank via line 4. As the water accumulates within the tank, the oil risesto the surface where it forms the separate oil phase 10. Water iswithdrawn from the tank through outlet 6 and then passed to a suitabledisposal zone such as a water injection well (not shown). As the oilrises to the surface of the water, it is automatically drained from thetank via conduit 16. As can be seen from an examination of FIG. 1, thebuoy 12 supports the conduit inlet 18 a fixed distance above theinterface 14. The oil thus is automatically drained from the tank at arate as is necessary to provide a constant thickness of the oil phase asdetermined by the elevation of the conduit inlet 18 above the oil-waterinterface. Accordingly, oil is prevented from accumulating within thetank in such large amounts such that it may ultimately overflow or bewithdrawn from the tank along with the water which is passed to thewater disposal zone. The oil phase will automatically be maintained atthe desired thickness regardless of fluctuations of the water levelwithin the tank. Thus, a continuous blanket of oil is provided over thewater. This blanket of oil serves as a corrosion protection cover whichprevents aeration of the water with oxygen from theatmosphere. Inaddition, as the water level within the tank varies, the oil leaves afilm along the sides of the tank, thus further protecting the tank fromcorrosion.

The oil blanket provides corrosion protection throughout that portion ofthe tank which is occupied by the water, and assuming that corrosion ofthe roof and upper wall surfaces of the tank is not a problem, the tankmay be open to the atmosphere through vent 5. However, a gas blanket maybe employed if corrosion above the portion of the tank protected by theoil phase 14 is severe. In this case, vent 5 will be normally closed andprovided with a vacuum-relief regulator valve (not shown). Thisregulator valve will function to open the vent if an excessive vacuumoccurs within the tank.

As illustrated in FIG. 1, the conduit 16 comprises a section of flexiblehose 16a which is connected to a nipple 1612 on the interface buoy 12.The buoy is of a density between the oil and water densities such thatit will float at the interface 14 when supporting the conduit. Thus, asthe height of the water column 9 is raised or lowered within the tank,the buoy will stay at the oil-water interface 14. The conduit inlet 18will therefore remain at a substantially fixed distance above theinterface 14 with only such minor variations as may occur when the buoyshifts position slightly due to changes in the amount of the conduit 16which is supported by the buoy.

As is best shown in FIG. 2, it is preferred that the buoy 12 have anannular configuration with the upper portion of conduit 16, i.e., nipple16b, extending through the interior opening. This configuration providesa stable support for the conduit and, in addition, is less likely tohang on deadwood" in the tank. The buoy may also be guided by wiresextending from bottom to top of the tank.

The buoy may be formed of a thin wall metal or plastic hollow memberwhich is loaded with a liquid such as water, and/or has weights added,in order to give it the proper density. If loaded with liquid, the buoyshould be substantially full in order to avoid instability.

It is desirable in practicing the invention to vary the rate of waterwithdrawal from the tank in order to move the oil phase verticallythroughout a substantial portion of the tank so that the sides of thetank within this interval are repeatedly contacted with oil. Thus, in apreferred embodiment of the invention, the tank is provided with meansfor alternately allowing the tank to fill with water to a predeterminedupper level and thereafter draining water from the tank until the waterreaches a predetermined lower level within the tank. More particularly,and with further reference to FIG. 1, the tank is provided with upperand lower level sensing means of any suitable type. In the embodimentshown, the level sensing means take the form of upper and lower floatswitches 20 and 22, respectively. These switches are connected throughsuitable control circuitry indicated by broken lines 20a and 22a to apump 24 located in the water outlet line 6.

As illustrated in FIG. I, the tank is shown in a filling cycle in whichwater is allowed to accumulate in the tank. In this cycle, pump 24 isshut off such that no water is withdrawn through line 6. The pumpremains off until the water rises to the level at which float switch 20is actuated. Pump 24 then is started and functions to withdraw waterfrom the tank at a rate greater than the rate of introduction throughline 4. The pump continues operating until the water reaches the lowerlevel at which float switch 22 is activated to shut down the pump.Thereafter, the pump remains off until the water again reaches a levelsufficient to actuate float switch 20. As will be recognized from theforegoing description, the oil level will be repeatedly moved throughoutthe section of the tank between the float switches 20 and 22, thusproviding a protective film which will retard corrosion of the tank.

It is to be recognized that the water may be withdrawn from the tank bygravity drainage. In this case the float switches 20 and 22 may beemployed to regulate a valve (not shown) in line 6 rather than pump 24.

In addition, the inlet line 4 may be provided with a valve (not shown)which is regulated by switches 20 and 22 in order to close off the flowof water into the tank while it is draining. Also, the float switchesmay be responsive either directly to the oil-water interface 14 or tothe top of the oil blanket 10. In the latter case, particular careshould be taken to locate switch 22 above the water outlet 6 by adistance greater than the thickness of oil layer 10 in order to ensurethat oil is not withdrawn through line 6. The water inlet line shouldopen into the tank at a position below the oil-water interface 14 inorder to avoid agitation of the oil phase. Thus, it is preferred tolocate the open end 4a of line 4 below float switch 22 so that the pointof water entry remains below the water surface when it reaches the lowerlevel within the tank.

What is claimed is:

1. In a method for the disposal of water, the steps comprismg:

introducing water containing oil into a surge tank and accumulating saidwater within said tank whereby it separates into a lower water phase andan upper oil phase,

withdrawing water from said water phase within said tank at a firstlocation below the interface of said oil and water phases underconditions such that the height of said interface varies within saidtank,

separately withdrawing oil from said oil phase within said tank througha conduit having an inlet supported above said oil-water interface by abuoy having a density between the densities of oil and water wherebysaid buoy floats at the oil-water interface and the oil is withdrawnthrough said conduit inlet at a rate to maintain the thickness of saidoil phase constant as the height of said interface varies, and

intermittently withdrawing said water from said tank at a ratesufficient to lower said interface to a level adjacent the bottom ofsaid tank and thereafter allowing water to accumulate in said tank untilsaid interface reaches a level adjacent the top of said tank.

2. In a water-disposal system, the combination comprising:

a surge tank adapted to receive water containing oil which separatesinto a lower water phase and an upper oil phase within said tank,

means for withdrawing water from said tank,

a conduit extending from the interior to the exterior of said tank forthe separate withdrawal of oil therefrom, said conduit having an inletin fluid communication with the interior of said tank,

means for maintaining said conduit inlet spaced a fixed distance abovethe interface between said oil and water phases whereby the thickness ofsaid oil phase remains constant as the elevation of said interfacevaries, said lastnamed means comprising a buoy to which said conduit issecured, said buoy having a density between the densities of oil andwater and being adapted to float at the interface between the oil andwater phases whereby said conduit inlet is supported above the interfaceby said buoy at said fixed distance, and

means for intermittently allowing said tank to fill with water to apredetermined upper level therein and thereafter emptying water fromsaid tank to a predetennined lower level therein.

3. The system of claim 2 further comprising a water inlet line openinginto said tank at a location below said predetermined lower level.

2. In a water-disposal system, the combination comprising: a surge tankadapted to receive water conTaining oil which separates into a lowerwater phase and an upper oil phase within said tank, means forwithdrawing water from said tank, a conduit extending from the interiorto the exterior of said tank for the separate withdrawal of oiltherefrom, said conduit having an inlet in fluid communication with theinterior of said tank, means for maintaining said conduit inlet spaced afixed distance above the interface between said oil and water phaseswhereby the thickness of said oil phase remains constant as theelevation of said interface varies, said last-named means comprising abuoy to which said conduit is secured, said buoy having a densitybetween the densities of oil and water and being adapted to float at theinterface between the oil and water phases whereby said conduit inlet issupported above the interface by said buoy at said fixed distance, andmeans for intermittently allowing said tank to fill with water to apredetermined upper level therein and thereafter emptying water fromsaid tank to a predetermined lower level therein.
 3. The system of claim2 further comprising a water inlet line opening into said tank at alocation below said predetermined lower level.